roll: third_party/pigweed pw_async2: Experimental Future type

Defines a standard `Future` interface for a pending asynchronous
operation. This is experimental and should not be used until stable.

Original-Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/321172
Presubmit-Verified: CQ Bot Account <pigweed-scoped@luci-project-accounts.iam.gserviceaccount.com>
Original-Revision: 3959870dd94ecd688c531544a32b71bb8efe1101

Rolled-Repo: https://pigweed.googlesource.com/pigweed/pigweed
Rolled-Commits: 2fb733cb5103f9..3959870dd94ecd
Roll-Count: 1
Roller-URL: https://cr-buildbucket.appspot.com/build/8703004592214326785
GitWatcher: ignore
CQ-Do-Not-Cancel-Tryjobs: true
Change-Id: I9b805abd0ab106c1a10dabe8a413886c356ea0b3
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/examples/+/324454
Lint: Lint 🤖 <android-build-ayeaye@system.gserviceaccount.com>
Bot-Commit: Pigweed Roller <pigweed-roller@pigweed-service-accounts.iam.gserviceaccount.com>
Commit-Queue: Pigweed Roller <pigweed-roller@pigweed-service-accounts.iam.gserviceaccount.com>
diff --git a/third_party/pigweed b/third_party/pigweed
index 2fb733c..3959870 160000
--- a/third_party/pigweed
+++ b/third_party/pigweed
@@ -1 +1 @@
-Subproject commit 2fb733cb5103f98af81d2bfedfd55e2eca7a8d52
+Subproject commit 3959870dd94ecd688c531544a32b71bb8efe1101